Fleet Angle

For proper spooling, and to prevent excessive wear on the drum grooves, the angle at which the rope leads to the drum, called the fleet angle, must be within controlled limits. Figures 4.2-6 and 4.2-7 present fleet angle definitions and recommended fleet angles.

Fleet angle should be within 1 to 2 degrees for smooth drums and not more than 1 1/4 degrees for groove drums. If the fleet angle is too small, it will result in considerable vibration, causing rope to pile up against the drum flange. This damages the rope and the equipment. If the fleet angle is too large, the rope will rub against the flanges of the sheave groove or be crushed on the drum. When it is not possible to place a lead sheave at the required distance from the drum, a pivoted block or fleeting sheave is used. A fleeting sheave is placed on a horizontal shaft, which allows the sheave to move laterally.

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Figure 4.2-7 Recommended Fleet Angles

Figure 4.2-8 Matching of Ropes and Sheaves 4.2.4 Sheaves

Sheaves are used to change travel direction of the wire ropes. Sheaves assembled in multiples form blocks that provide the required mechanical advantage. The condition and contour of sheave grooves play a major role in the useful life span of the wire rope and sheave. As discussed earlier, a 2-degree fleet angle is recommended. However, constant misalignment causes the rope to rub the sides of the groove, resulting in wear of the rope and sheave. The grooves must be smooth and slightly larger than the rope to prevent it from being pinched or jammed in the groove. Table 4.2-3 shows the sheave groove tolerances.

Table 4.2-3 Sheave Groove Tolerances

The bottom of the groove should have an arc of support of at least 120 to 150 degrees, and the sides of the groove should be tangent to the arc. Figure 4.2-8 shows a proper arc of support for rope by a sheave. In addition, the figure shows the effects of too large and too small diameter of rope on the sheave.

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If the groove diameter is too large, the rope will not be properly supported and will tend to flatten and become distorted. Figure 4.2-9 shows the effect of an improperly matched sheave and wire rope.

4 . 2 A N C I L L A R Y C O M P O N E N T S

Figure 4.2-9 Effects of an Improper Match Between Rope and Sheave

Figure 4.2-10 shows a badly damaged sheave and how sheave grooves are to be checked for proper wire rope size.

The depth of the sheave grooves should be at least 1-1/2 times the rope’s diameter, and the tapered side walls of the grooves should not make an angle greater than 18 degrees with respect to the centerline. The flange corners should be rounded, and the rims should run true about the axis of rotation. The bearings should be permanently lubricated or be equipped with a means for lubrication. Figure 4.2-11 shows the sheave requirements.

Figure 4.2-11 Sheave Measurements

Sheave and drum diameters have a direct bearing on rope life. One of the fastest ways to ruin a wire rope is to operate it over too small a sheave. All wire ropes operating over sheaves and drums are subject to cyclic bending stresses. The magnitude of stress depends on the ratio of the diameter of the sheave or drum to the diameter of the wire rope (D/d). Table 4.2-4 suggests the minimum D/d ratios for various rope construction.

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Table 4.2-4 Sheave Diameter Factors

4 . 2 A N C I L L A R Y C O M P O N E N T S

The ratio of sheave and drum to rope diameter for cranes and derricks—stipulated by ASME standards—is fixed and does not vary with rope life parameters. The winding drum and upper block sheave diameters will not be less than 18 times the wire rope diameter, while the lower block sheave diameter will not be less than 16. These ratios apply to the load hoisting systems of construction cranes and derricks. The ratios for overhead and industrial cranes are more conservative.

There is no minimum sheave or drum diameter that prevents a hoisting mechanism from operating. However, as shown in Figure 4.2-12, a wire rope’s life decreases with decreasing sheave and drum diameters.

Relative bending life factors show that rope construction has a direct relation to the bending stress concerning longer service of the rope. For example, changing from 6 x 25 filler wire (FW) with a factor of 1 to a 6 x 36 Warrington Seale (WS) with a factor of 1.15 means that the service life of the rope could be increased by 15 percent.

Figure 4.2-12 Service Life of Wire Rope

Example:

A rope working with a D/d ratio of 26 has a relative service life of 17. If the same rope works over a sheave that has a D/d ratio of 35, the relative service life increases to 32, which means an 88 percent increase in service life.

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4.2.5 Blocks

A block is a frame that encloses one or more sheaves and is provided with a hook or some other means that allows attachment to cargo or to a fixed anchor point. The purpose of a block is twofold. First, it is used to change direction of a wire rope line. Second, when used in pairs, blocks increase mechanical advantage by allowing the use of multiple parts of line. Blocks range in size from several pounds capacity to hundreds of tons.

There are three basic types of blocks: crane, snatch, and wire rope (construction or fixed) blocks. Snatch blocks refer to a group of intermittent service blocks that jerk or snatch their load over comparatively short distances. Snatch blocks are characterized by a side-opening plate that facilitates threading the wire rope through the block. As opposed to a snatch block, a crane block is required to perform long lifts under continuous service conditions. Crane blocks are characterized by multiple large diameter, long service life sheaves, and the addition of cheek plate weights to the block side frames to increase overhaul weight. Crane blocks

typically are ouffitted with a swivel hook that allows the cargo to be rotated without fouling the multiple parts of reeving. Fixed blocks or construction blocks are typically used as upper blocks in multi-part reeving arrangements in derricks or material hoists. As such, they have large diameter multiple sheaves like crane blocks but the lack the additional cheek plate weights required for overhaul.

A block consists of a shell (or side plates), a center pin, and an end fitting. There are a variety of end fittings such as hooks, shackles, and clevices that facilitate attachment of the block to the cargo or to a fixed anchorage. Blocks are also equipped with a becket or mouse ear whereby the end of the rope line is affixed to the block. The sheaves of the block transmit the load from the wire rope to the center pin and then to the shell straps or side plates. Figures 4.2- 13, 4.2-14, and 4.2-15 provide illustration of wire rope blocks, crane and hook blocks, wire rope blocks, and snatch blocks.

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Figure 4.2-14 Typical Crane and Hook Block

Figure 4.2-15 Various Wire Rope Blocks and Snatch Blocks